Description:
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When calling empty() with a return value from function, it says:

Can't use method return value in write context

It would be definitely less confusing if it said something like:

empty() accepts only variables as a parameter.

Reproduce code:
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<?php
empty(trim(""));
?>

Expected result:
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Fatal error: empty() accepts only variables as a parameter in  ... on line 2

Actual result:
--------------
Fatal error: Can't use method return value in write context in ... on line 2